The most common optical connection is between an optical fiber end and a collimating lens, such as a graded index (GRIN) lens. A common coupler system comprises a sleeve in which a fiber end within a supporting ferrule is joined to a lens by attaching the sides of the ferrule and the lens to the interior bore of the sleeve. Manufacturing tolerances of, for instance, the lens diameter, the ferrule diameter, or the dimension and position of the bore within the ferrule, introduce variations which make this type of coupler difficult to use to achieve accurate alignment. The sleeve must have a large enough internal diameter to accommodate a ferrule or lens at its largest tolerance. As a result, smaller elements may unintentionally become tilted within the sleeve. The fiber end within the ferrule is not always concentric. If the ferrule internal diameter is too large for the fiber, or the ferrule bore is not perfectly centred, it becomes necessary to adjust, or tune, the alignment of the fiber to the lens. This is difficult within the confines of the coupling sleeve. Temperature changes in the environment also cause different elements in the sleeve coupler to respond differently, further affecting the quality of the coupling. In many instances, it is preferred to have the fiber aligned at a port on the lens that is disposed adjacent to, rather than aligned with, the optical axis of the lens. This is not possible within a sleeve coupler.
Another common practice is to couple an optical fiber to a lens, such as a graded index (GRIN) lens, by placing the optical fiber into a ferrule or fiber tube and joining an end face of the tube directly to a face of the lens by applying a layer of epoxy between them. A direct ferrule to lens epoxy connection facilitates alignment of the coupled elements and provides a uniform joint. The end faces of the lens and ferrule can be formed to planar surfaces reliably to prevent unwanted tilt angle at the coupling. Positioning the fiber adjacent to the optical axis of the lens is possible without the confining sleeve coupler. Also, relatively easy alignment tuning is possible to compensate for eccentric fiber positioning within the ferrule. However, epoxy is not completely transparent or uniform to transmission. Transmission of light though the epoxy layer results in some loss.
To couple an optical fiber end to an optical element, such as a lens, there are three positioning variables: the spacing between the elements, the coupling point relative to the optical axis, and the coupling angle. Common coupling techniques do not provide a convenient method of adjusting the spacing between the elements, or the coupling angle.
It is often desired to make a coupling with a precise predetermined distance between elements, which is not possible with a direct epoxy connection. Typically this is accomplished by fixing the elements with the desired separation between them on a substrate with adhesive. It is difficult to establish an exact separation, or alignment and angular positioning by this method.
In addition to a controlled coupler spacing, the angle of coupling needed varies. A correct coupling angle is critical in some configurations for a low loss coupling. It has been noted in co-pending application Ser. No. 09/015,325 filed on Jan. 29, 1998 assigned to the same entity as the present application, that substantial coupling losses may occur between an input port on a first GRIN lens, or other type of collimating lens, and an output port of a second collimating lens, when the input and output ports are disposed adjacent the optical axes of the two collimating lenses, and when the distance or gap between the lenses is significant. An optical element, such as a filter, may cause a gap between the GRIN lenses, with the result that the element causes a beam propagating from the input port through the first GRIN lens to be shifted as it traverses the element towards the output port and enters the second lens at an offset to the optical axis of the lens. This beam shift downward results in the output beam exiting upward, no longer parallel to the optical axis. Hence, more efficient coupling can be achieved if the fiber at the output of the lens assembly is provided at an appropriate angle with respect to the optical axis of the lens.
It is desired to provide a coupling system which provides a reliable means for coupling between optical elements. It is further desired to provide a coupling system which is unimpeded by transmission through an epoxy layer. It is also desired to provide a coupling system which can accurately control the spacing between optical elements, and provide adjustment to the alignment and the angle of coupling.